Semiconductor devices, including integrated circuit devices such as boot-type devices, are tested prior to shipment to ensure quality control. In the case of boot-type devices known as flash memory devices, the testing can include parametric tests, including testing for short circuits, open circuits, various leakage tests, and various signature tests. Also, the testing can include various performance or functional tests, such as speed tests and erase time tests. Of importance to the present invention is testing a device for erase time.
Specifically, to determine whether a device can be erased adequately, each sector in the device is programmed, and then the device is erased, sector by sector, with the time period required for erasure being recorded for each sector. In the event that any leakage is detected after an erase operation, a test known as Auto Program Disturb Erase (APDE) is performed to cure the leakage, and then the APDE time period is also recorded. A test limit is then calculated that is essentially the average (or, e.g., 1.5 times the average) of the erase time periods and APDE time periods of all the sectors. If the time periods required for any sector exceed the test limit, the device is rejected.
As recognized by the present invention, for test purposes it has been assumed that flash memory devices have equally-sized sectors, that is, all the sectors to be tested of flash memory devices are assumed to be of uniform size. Accordingly, a single test limit has been used against which the erase/APDE times of all sectors are compared.
The present invention understands, however, that flash memory devices can have some sectors of one uniform size (typically of a larger size) and other sectors (boot sectors) of other, non-uniform sizes (typically of smaller sizes and, hence, colloquially referred to as “baby sectors”). In these devices, the smaller baby sectors should have shorter erase times than the larger uniform sectors. Nonetheless, current test procedures still use a single test limit against which all sectors are compared. The present invention recognizes that when a single test limit is used, it is based on times derived from both the uniform sectors and baby sectors and, hence, might be set too low for uniform sectors, thus resulting in a large number of false rejects. Having made the above critical observations, the present invention provides the below-solutions to one or more of the observations.